Electrostatic speakers are attracting attention as speakers capable of generating sound with high straight forwardness by emitting plane waves.
An electrostatic speaker has a structure in which, on each side of a sheet-like oscillator having conductivity, a sheet-like electrode having conductivity is disposed with an insulating spacer held therebetween. The electrode is required to be equipped with numerous through-holes passing through the inside and outside faces thereof and serving as air moving paths so as not to obstruct the oscillation of the oscillator, and the electrode is made of, for example, a cloth woven from conductive fibers or a punching metal sheet.
When a direct-current bias voltage is applied between the oscillator and each of the two electrodes, and an alternate-current voltage drive signal corresponding to a sound waveform is applied between the two electrodes, a drive force corresponding to the drive signal is generated between the oscillator and the electrode in accordance with Coulomb's law, and the oscillator oscillating between the two electrodes by virtue of the drive force generates sound pressure. As a result, sound corresponding to the sound waveform is emitted.
For the purpose oscillating the oscillator between the two electrodes, each of the two electrodes is required to be disposed so as to be isolated from the oscillator. Hence, a cushioning material layer having insulation property, air permeability and elasticity is disposed between the oscillator and each of the two electrodes. As the cushioning material layer, for example, a nonwoven cloth made of PET (polyethylene terephthalate) can be used.
In the case that a laminated body formed of a plurality of sheet-like members, in which the cushioning material layer is disposed on each side of the oscillator and the electrode is disposed on each outside thereof as describe above, is exposed directly to the outside, the user may touch the electrode and receive an electric shock or dust may attach to the electrode or the oscillator, thereby inducing discharge or leakage between the electrode and the oscillator. For the purpose of avoiding these problems, a cover layer is usually disposed on the outside of each of the two electrodes. As the cover, for example, a PET film can be used. Since the PET film also has waterproofness, the film can prevent intrusion of not only dust but also liquid and moisture to the inside and can also prevent corrosion or the like of the electrodes and the conductive layer of the oscillator.
Since the above-mentioned cover layer is easily broken when the layer collides with a pen tip or the like or makes contact with a high-temperature lamp or the like, an outside cover having flame retardant property and cushioning property is disposed in some cases further outside the above-mentioned cover layer. As the outside cover to be used for such a purpose, for example, a flame-retardant nonwoven cloth can be used.
The above-mentioned cover layer is required to be formed into a bag-like shape to protect the electrodes and the oscillator to be accommodated therein. At the time, the cover layer is required to be air tight to hold therein an air layer required for the oscillation of the oscillator and to prevent intrusion of moisture or the like from the outside to the inside.
As widely spread techniques for fixing a plurality of laminated synthetic resin sheets so that the outer edge regions thereof are sealed with one another, a method in which the sheets are bonded using an adhesive and a method in which welding is performed between the sheets by heating and melting and then by cooling the synthetic resin, that is, heat sealing (heat welding), are available.
In the case of using an adhesive, a process of applying the adhesive to one sheet, quickly positioning the other sheet and disposing the other sheet on the sheet on which the adhesive has been applied, pressing the adhesion portions thereof and waiting for the adhesive to be hardened is performed, whereby the process is complicated and takes time. Furthermore, for the purpose of fixing all the n sheets of the laminated body using the adhesive, the above-mentioned process is required to be performed (n−1) times.
On the other hand, in the case of heat sealing, a belt-like heater is pressed against previously positioned and laminated sheets to heat them, whereby fixing between the sheets is completed. The time required for the melted synthetic resin to be cooled and hardened is generally shorter than the hardening time of the adhesive, and the process required for fixing all the n sheets of the laminated body is the same as the process for fixing two sheets, whereby the heater is merely required to be pressed once against the sheets to heat them. Hence, generally, heat sealing is low in cost as a method for fixing the synthetic resin sheets of the laminated body to one another in comparison with the case in which the adhesive is used.
A technique for performing welding between synthetic resin sheets by heat sealing is disclosed, for example, in Patent Document 1. In Patent Document 1, a technique has been proposed to reduce sealing defects in heat sealing by specifying the difference between the crystal melting points of two synthetic resin layers to be welded to each other, the thickness of the synthetic resin layer having a lower crystal melting point, and the optimal value of the thickness ratio of the layers.